Multiplex Detection
Large-scale multiplex analysis of highly polymorphic loci is needed for practical identification of individuals, e.g., for paternity testing and in forensic science (Reynolds et al., Anal. Chem., 63:2–15 (1991)), for organ-transplant donor-recipient matching (Buyse et al., Tissue Antigens, 41:1–14 (1993) and Gyllensten et al., PCR Meth. Appl, 1:91–98 (1991)), for genetic disease diagnosis, prognosis, and pre-natal counseling (Chamberlain et al., Nucleic Acids Res., 16:11141–11156 (1988) and L. C. Tsui, Human Mutat., 1:197–203 (1992)), and the study of oncogenic mutations (Hollstein et al., Science, 253:49–53 (1991)). In addition, the cost-effectiveness of infectious disease diagnosis by nucleic acid analysis varies directly with the multiplex scale in panel testing. Many of these applications depend on the discrimination of single-base differences at a multiplicity of sometimes closely spaced loci.
A variety of DNA hybridization techniques are available for detecting the presence of one or more selected polynucleotide sequences in a sample containing a large number of sequence regions. In a simple method, which relies on fragment capture and labeling, a fragment containing a selected sequence is captured by hybridization to an immobilized probe. The captured fragment can be labeled by hybridization to a second probe which contains a detectable reporter moiety.
Another widely used method is Southern blotting. In this method, a mixture of DNA fragments in a sample is fractionated by gel electrophoresis, then fixed on a nitrocellulose filter. By reacting the filter with one or more labeled probes under hybridization conditions, the presence of bands containing the probe sequences can be identified. The method is especially useful for identifying fragments in a restriction-enzyme DNA digest which contains a given probe sequence and for analyzing restriction-fragment length polymorphisms (“RFLPs”).
Another approach to detecting the presence of a given sequence or sequences in a polynucleotide sample involves selective amplification of the sequence(s) by polymerase chain reaction. U.S. Pat. No. 4,683,202 to Mullis, et al. and R. K. Saiki, et al., Science 230:1350 (1985). In this method, primers complementary to opposite end portions of the selected sequence(s) are used to promote, in conjunction with thermal cycling, successive rounds of primer-initiated replication. The amplified sequence(s) may be readily identified by a variety of techniques. This approach is particularly useful for detecting the presence of low-copy sequences in a polynucleotide-containing sample, e.g., for detecting pathogen sequences in a body-fluid sample.
More recently, methods of identifying known target sequences by probe ligation methods have been reported. U.S. Pat. No. 4,883,750 to N. M. Whiteley, et al., D. Y. Wu, et al., Genomics 4:560 (1989), U. Landegren, et al., Science 241:1077 (1988), and E. Winn-Deen, et al., Clin. Chem. 37:1522 (1991). In one approach, known as oligonucleotide ligation assay (“OLA”), two probes or probe elements which span a target region of interest are hybridized to the target region. Where the probe elements basepair with adjacent target bases, the confronting ends of the probe elements can be joined by ligation, e.g., by treatment with ligase. The ligated probe element is then assayed, evidencing the presence of the target sequence.
In a modification of this approach, the ligated probe elements act as a template for a pair of complementary probe elements. With continued cycles of denaturation, hybridization, and ligation in the presence of pairs of probe elements, the target sequence is amplified linearly, allowing very small amounts of target sequence to be detected and/or amplified. This approach is referred to as ligase detection reaction. When two complementary pairs of probe elements are utilized, the process is referred to as the ligase chain reaction which achieves exponential amplification of target sequences. F. Barany, “Genetic Disease Detection and DNA Amplification Using Cloned Thermostable Ligase,” Proc. Nat'l Acad. Sci. USA, 88:189–93 (1991) and F. Barany, “The Ligase Chain Reaction (LCR) in a PCR World,” PCR Methods and Applications, 1:5–16 (1991).
Another scheme for multiplex detection of nucleic acid sequence differences is disclosed in U.S. Pat. No. 5,470,705 to Grossman et. al. where sequence-specific probes, having a detectable label and a distinctive ratio of charge/translational frictional drag, can be hybridized to a target and ligated together. This technique was used in Grossman, et. al., “High-density Multiplex Detection of Nucleic Acid Sequences: Oligonucleotide Ligation Assay and Sequence-coded Separation,” Nucl. Acids Res. 22(21):4527–34 (1994) for the large scale multiplex analysis of the cystic fibrosis transmembrane regulator gene.
Jou, et. al., “Deletion Detection in Dystrophia Gene by Multiplex Gap Ligase Chain Reaction and Immunochromatographic Strip Technology,” Human Mutation 5:86–93 (1995) relates to the use of a so called “gap ligase chain reaction” process to amplify simultaneously selected regions of multiple exons with the amplified products being read on an immunochromatographic strip having antibodies specific to the different haptens on the probes for each exon.
There is a growing need (e.g., in the field of genetic screening) for methods useful in detecting the presence or absence of each of a large number of sequences in a target polynucleotide. For example, as many as 400 different mutations have been associated with cystic fibrosis. In screening for genetic predisposition to this disease, it is optimal to test all of the possible different gene sequence mutations in the subject's genomic DNA, in order to make a positive identification of “cystic fibrosis”. It would be ideal to test for the presence or absence of all of the possible mutation sites in a single assay. However, the prior-art methods described above are not readily adaptable for use in detecting multiple selected sequences in a convenient, automated single-assay format.
Solid-phase hybridization assays require multiple liquid-handling steps, and some incubation and wash temperatures must be carefully controlled to keep the stringency needed for single-nucleotide mismatch discrimination. Multiplexing of this approach has proven difficult as optimal hybridization conditions vary greatly among probe sequences.
Developing a multiplex PCR process that yields equivalent amounts of each PCR product can be difficult and laborious. This is due to variations in the annealing rates of the primers in the reaction as well as varying polymerase extension rates for each sequence at a given Mg2+ concentration. Typically, primer, Mg2+, and salt concentrations, along with annealing temperatures are adjusted in an effort to balance primer annealing rates and polymerase extension rates in the reaction. Unfortunately, as each new primer set is added to the reaction, the number of potential amplicons and primer dimers which could form increase exponentially. Thus, with each added primer set, it becomes increasingly more difficult and time consuming to work out conditions that yield relatively equal amounts of each of the correct products.
Allele-specific PCR products generally have the same size, and an assay result is scored by the presence or absence of the product band(s) in the gel lane associated with each reaction tube. Gibbs et al., Nucleic Acids Res., 17:2437–2448 (1989). This approach requires splitting the test sample among multiple reaction tubes with different primer combinations, multiplying assay cost. PCR has also discriminated alleles by attaching different fluorescent dyes to competing allelic primers in a single reaction tube (F. F. Chehab, et al., Proc. Natl. Acad. Sci. USA, 86:9178–9182 (1989)), but this route to multiplex analysis is limited in scale by the relatively few dyes which can be spectrally resolved in an economical manner with existing instrumentation and dye chemistry. The incorporation of bases modified with bulky side chains can be used to differentiate allelic PCR products by their electrophoretic mobility, but this method is limited by the successful incorporation of these modified bases by polymerase, and by the ability of electrophoresis to resolve relatively large PCR products which differ in size by only one of these groups. Livak et al., Nucleic Acids Res., 20:4831–4837 (1989). Each PCR product is used to look for only a single mutation, making multiplexing difficult.
Ligation of allele-specific probes generally has used solid-phase capture (U. Landegren et al., Science, 241:1077–1080 (1988); Nickerson et al., Proc. Natl. Acad. Sci. USA, 87:8923–8927 (1990)) or size-dependent separation (D. Y. Wu, et al., Genomics, 4:560–569 (1989) and F. Barany, Proc. Natl. Acad. Sci., 88:189–193 (1991)) to resolve the allelic signals, the latter method being limited in multiplex scale by the narrow size range of ligation probes. Further, in a multiplex format, the ligase detection reaction alone cannot make enough product to detect and quantify small amounts of target sequences. The gap ligase chain reaction process requires an additional step—polymerase extension. The use of probes with distinctive ratios of charge/translational frictional drag for a more complex multiplex will either require longer electrophoresis times or the use of an alternate form of detection.
The need thus remains for a rapid single assay format to detect the presence or absence of multiple selected sequences in a polynucleotide sample.
Microsatellite Analysis
Tandem repeat DNA sequences known as microsatellites represent a very common and highly polymorphic class of genetic elements within the human genome. These microsatellite markers containing small repeat sequences have been used for primary gene mapping and linkage analysis. Weber, J. L. et al., Am. J. Hum. Genet. 44: 388–396 (1989); Weissenbach, J. et al., Nature (London) 359: 794–800 (1992). PCR amplification of these repeats allows rapid assessment for loss of heterozygosity and can greatly simplify procedures for mapping tumor suppressor genes. Ruppert, J. M., et al., Cancer Res. 53: 5093–94 (1993); van der Riet, et al., Cancer Res. 54: 1156–58 (1994); Nawroz, H., et al., Cancer Res. 54: 1152–55 (1994); Cairns, P., et al., Cancer Res. 54: 1422–24 (1994). More recently, they have been used to identify specific mutations in certain inherited disorders including Huntington disease, fragile X syndrome, myotonic dystrophy, spinocerebellar ataxia type I, spinobulbar muscular atrophy, and hereditary dentatorubral-pallidoluysian atrophy. The Huntington's Disease Collaborative Research Group Cell 72: 971–83 (1993); Kremer, E. J., et al., Science 252: 1711–14 (1991); Imbert, G., et al., Nat. Genet. 4: 72–76 (1993); Orr, H. T., et al., Nat. Genet. 4: 221–226 (1993); Biancalana, V., et al., Hum. Mol. Genet. 1: 255–258 (1992); Chung, M. -Y., et al., Nat. Genet. 5: 254–258 (1993); Koide, R., et al., Nat. Genet. 6: 9–13 (1994). These inherited disorders appear to arise from the expansion of trinucleotide repeat units within susceptible genes. A more widespread microsatellite instability, demonstrated by expansion or deletion of repeat elements in neoplastic tissues, was first reported in colorectal tumors. Peinado, M. A., et al. Proc. Natl. Acad. Sci. USA 89: 10065–69 (1992); Ionov, Y., Nature (London) 363: 558–61 (1993); Thibodeau, S. N., et al., Science 260: 816–819 (1993) and later in several other tumor types (Risinger, J. I., Cancer Res. 53: 5100–03 (1993); Han, H. -J., et al., Cancer Res. 53: 5087–89 (1993); Peltomäki, P., Cancer Res. 53: 5853–55 (1993); Gonzalez-Zulueta, M., et al., Cancer Res. 53: 5620–23 (1993); Merlo, A., et al., Cancer Res. 54: 2098–2101 (1994)). In hereditary nonpolyposis colorectal carcinoma patients, this genetic instability is apparently due to inherited and somatic mutations in mismatch repair genes. Leach, F., et al., Cell 75: 1215–1225 (1993); Fishel, R., et al., Cell 75: 1027–38 (1993); Papadopoulos, N., et al., Science 263: 1625–29 (1994); Bronner, C. E., et al., Nature (London) 368: 258–61 (1994).
PCR is commonly used for microsatellite analysis in identifying both the appearance of new polymorphisms and the loss of heterozygosity in cancer detection. L. Mao, et. al., “Microsatellite Alterations as Clonal Markers for the Detection of Human Cancer,” Proc. Nat'l Acad. Sci USA 91(21): 9871–75 (1994); L. Mao, et. al., “Molecular Detection of Primary Bladder Cancer by Microsatellite Analysis,” Science 271:659–62 (1996); D. Radford, et. al., “Allelotyping of Ductal Carcinoma in situ of the Breast: Detection of Loci on 8p, 13q, 161, 17p and 17q,” Cancer Res. 55(15): 3399–05 (1995). In using PCR for such purposes, each PCR reaction is run individually and separated on a sequencing gel.
Although these references demonstrate that PCR has application to diagnosis and prognosis of certain cancers, this type of analysis is deficient, because it does not permit a high throughput and requires size separation. In addition, there are problems with PCR slippage, causing researchers to shift to tri-, tetra-, and higher nucleotide repeat units, making cancer detection more difficult.
Microsatellite markers have also been used for colon cancer detection (L. Cawkwell, et. al., “Frequency of Allele Loss of DCC, p53, RB1, WT1, NF1, NM23, and APC/MCC in Colorectal Cancer Assayed by Fluorescent Multiplex Polymerase Chain Reaction,” Br. J. Cancer 70(5): 813–18 (1994)) and for genome mapping (P. Reed, et. al., “Chromosome-specific Microsatellite Sets for Fluorescent-Based, Semi-Automated Genome Mapping,” Nat. Genet. 7(3): 390–95 (1994)). However, the key to such multiplex processes is the ability to perform them in a single reaction tube. Conventional multiplex microsatellite marker approaches require careful attention to primer concentrations and amplification conditions. Although PCR products can be pooled in sets, this requires a prerun on agarose gels to insure that the mixture has about equal amounts of DNA in each band.
Human Identification
PCR has also been used for human identification, such as paternity testing, criminal investigations, and military personnel identification. A. Syvanen et. al., “Identification of Individuals by Analysis of Biallelic DNA Markers, Using PCR and Solid-Phase Mini-Sequencing” Am. J. Hum. Genet. 52(1): 46–59 (1993) describes a mini-sequencing approach to human identification. The technique requires PCR amplification of individual markers with at most 4 PCR reactions being carried out at a time in a single PCR tube. Mini-sequencing is carried out to determine individual polymorphisms.
Coupled Processes
G. Deng, et. al., “An Improved Method of Competitive PCR for Quantitation of Gene Copy Number,” Nucl. Acids Res. 21: 4848–49 (1993) describes a competitive PCR process. Here, two PCR steps are utilized with different sets of primers being used for each gene and its equivalent standard.
T. Msuih, et. al., “Novel, Ligation-Dependent PCR Assay for Detection of Hepatitis C. Virus in Serum,” J. Clin Microbio. 34: 501–07 (1996) and Y. Park, et. al., “Detection of HCV RNA Using Ligation-Dependent Polymerase Chain Reaction in Formalin-Fixed Paraffin-Embedded Liver Tissue” (submitted) describe the use of a LDR/PCR process in work with RNA.